CUC as per marine tanks don't really exist in a tropical set up.
There are some fish that eat algae, with variable success, such as the bristlenose mentioned, and some shrimps that will do a bit of tidying, but no real CUC.
Out of curiosity, what do you mean by 2 male gouramis? IME they tend to be fairly territorial when you have multiple males and the betta's have a similar mindset. You may have a fight on your hands with that combo.